1. Field of the Invention
The present invention relates to a microgyroscope for sensing an angular velocity. More particularly, the present invention relates to a microgyroscope not readily affected by external disturbances, which is capable of synchronizing a sensing electrode and a corresponding sensing mass in a same direction and/or with a same resonant frequency in a sensing direction thereby eliminating unnecessary signal output made due to an external translational acceleration caused by disturbances, such as noise, shock, and the like.
2. Description of the Related Art
A gyroscope is a sensing device that detects rotational angular velocity, and is currently in use as a core part for precision navigation in ships and airplanes. Recently, developments in micro-electromechanical system (MEMS) technology have enabled the application of a gyroscope in a navigation device of automobiles and as a hand-oscillation compensating device of high performance video cameras.
A gyroscope operates based on a Coriolis force, which acts on a mass in a third axis direction when the mass, which is oscillating or rotating in a first axis direction, is applied with a force rotating at a constant angular velocity from a second axis direction normal to the first axis direction. The angular velocity is detected by sensing a change in the displacement of the sensing mass and a capacitance change.
Referring to FIG. 1, a conventional microgyroscope 10 of MEMS technology is provided with an oscillating mass 12, i.e., oscillating mass Ma, moving at a resonant frequency fa by an oscillating direction elastic body 13 that has a predetermined damping force, or a damper 15 and oscillates in a horizontal direction, i.e., in an X-axis direction, a drive electrode 16 having drive combs 17 arranged between oscillating combs 14 of the oscillating mass 12 at predetermined intervals and secured on a wafer 11, a sensing mass 18, i.e., sensing mass Ms, oscillating together with the oscillating mass 12 by a sensing direction elastic body 19 that has a predetermined damping force, or a damper 23, and then with application of rotational force at a constant angular velocity, oscillating in a vertical direction, i.e., in a Y-axis direction at a resonant frequency fs, and a sensing electrode 22 having electrode combs 21 arranged between sensing combs 20 of the sensing mass 18 at predetermined intervals, and secured on the wafer 11.
The operation of the microgyroscope 10 constructed as above, will be explained below. First, as AC voltage is supplied to the drive electrode 16, the oscillating mass 12 and the sensing mass 18 oscillate in the X-axis direction by the oscillating and drive combs 14, 17 at the resonant frequency fa.
As the microgyroscope 10 is rotated by an external force at an angular velocity Ω, the oscillating mass 12 and the sensing mass 18 are subject to the Coriolis force in the Y-axis direction.
The degree of Coriolis acceleration is represented by:                                           y            ¨                    coriolis                =                  2          ⁢                                    Ω              z                        ⁡                          (              t              )                                ×                                    x              .                        ⁡                          (              t              )                                                          (        1        )            where       x    .    ⁡      (    t    )  is the differentiation of time with respect to the displacement of the oscillating mass 12 in the X-axis direction, and t is time.
By the Coriolis acceleration, the sensing mass 18 is oscillated in the Y-axis direction by the sensing direction elastic body 19. If the sensing mass 18 is displaced in the Y-axis direction by even a minute distance, e.g., from several tens of nanometers to several nanometers, a capacitance between the sensing combs 20 of the sensing mass 18 and the electrode combs 21 of the sensing electrode 22 varies. Accordingly, the voltage change thereof is detected as an angular velocity.
However, in addition to the angular velocity Ω, the microgyroscope 10 is equally exposed to external disturbances, such as noise or shock. If the microgyroscope is subject to such a disturbance, the sensing mass 18 is displaced due to a translational acceleration. The translational acceleration, particularly in the Y-axis direction, causes the sensing mass 18 to displace, and a subsequent sensing of unnecessary signals.
More specifically, the properties of the signals appearing during the vibration of the sensing mass 18 by the disturbance in the absence of input angular velocity Ω is expressed by:A cos ωot·cos ωst  (2) where ωa is a resonant frequency of the oscillating mass 12, ωs is a resonant frequency of the sensing mass 18, and A is an amplitude.
Separately expressing two frequency components based on the above equation (2) will render:1/2A[cos(ωo−ωa)t+cos(ωo+ωs)t]  (3) 
One of the two frequency components is removed as it is passed through a low pass filter of a signal sensing circuit. The other frequency component, which is 1/2A[cos(ωo−ωs)t, however, is not removed and thus remains even after having passed through the low pass filter. This is because the resonant frequency ωs of the sensing mass 18 is set higher than the resonant frequency ωo of the oscillating mass 12 during the designing process to maximize sensitivity, thereby rendering a relatively small difference between the frequencies ωo−ωs.
Accordingly, as shown in FIG. 2, unnecessary signals are detected when an external shock is applied to the microgyroscope 10.